The present invention relates to a bearing for use submerged in fluids at very low temperatures. In more detail, the present invention relates to a bearing with hardened rolling elements, hardened race and polymeric cage or retainer. In one embodiment the inventive bearing is used in a pump in which the drive source, or motor, is separated from the pump by a housing and is rotatably driven by the interaction of first and second magnets acting through the housing for use in pumping fluids at very cold temperatures, for instance, for use in pumping liquefied natural gas (LNG), which has a temperature of about xe2x88x92263xc2x0 F. (xe2x88x92164xc2x0 C.).
Liquefied natural gas (LNG) and other very low temperature fluids are of increasing commercial importance. There is, therefore, a need for increased facility in handling, storing, and transporting such liquids. For example, LNG is being increasingly utilized as an alternative fuel source for internal combustion engines. Governmental regulations require that LNG be transported at pressures of about 30 psi, but to decrease the amount of LNG that is evaporated or otherwise lost from a stationary storage tank, it is common to store the LNG at pressures of 150 psi. When xe2x80x9cbottledxe2x80x9d for use as the fuel for an internal combustion engine, it is common to pressurize the LNG to pressures as high as 220 psi. Of course each increase in pressure requires that the LNG be pumped into either a storage tank or into a fuel tank at the next higher pressure such that successful use of LNG as an alternative fuel depends, in effect, upon reliable, safe and energy efficient pumping of high volumes of such fluids at very low temperatures.
Bearings used submerged in very low temperature fluids often operate without a source of lubrication flowing with the fluid. Oil and grease or other normal lubricants will not function at the very low temperatures. Graphite and other solid friction reducing materials wash away in the fluid and can unacceptably contaminate the fluid.
Pumps presently used for pumping low temperature fluids all suffer from a variety of disadvantages and limitations which limit their life, require frequent maintenance, and otherwise decrease their utility. This is particularly true when the temperature of the fluid must be very low. For instance, pumps that are currently available for pumping LNG, wear out quickly and need frequent maintenance and particularly require frequent replacement of the seals. Heretofore known seal-less pumps have not provided a satisfactory solution to this problems. For instance, magnetic drive pumps known in the fluid pump art, are not reliable for use at very low temperatures. Bonding material utilized on the magnets at low temperature nevertheless fails at very low temperature. In the case of LNG, severe problems result from the almost complete lack of lubrication that is provided by the LNG passing through the pump. As a result bearings wear rapidly and need frequent replacement. Sometimes rapid bearing wear leads to catastrophic pump failure.
It is, therefore, an object of the present invention to provide a pump for use at very low temperatures which is not limited by the disadvantages of known pumps. More specifically, it is an object of the present invention to provide an improved bearing for a seal-less magnetic drive pump for use in pumping at very low temperatures.
Another object of the present invention is to provide a bearing useful at very low temperatures used herein to mean temperatures lower than about xe2x88x92100xc2x0 C. Yet another object of the present invention is to provide such a low temperature bearing which is self lubricated when used in a pump for pumping, high volumes of commonly utilized fluids as LNG and liquid nitrogen.
It is a further object to provide a bearing constructed for self lubrication when it is submerged in cryogenic fluid such as LNG and liquid nitrogen.
The objects, and the advantages, of the present invention are met by providing bearing that is useful in a magnetic drive pump for use in pumping fluids at very low temperature, below about xe2x88x92100xc2x0 C. and more particularly for pumping cryogenic fluids at temperatures below about xe2x88x92150xc2x0 C. The pump has a back plate with a rotatable shaft journaled therein. The rotatable shaft is journaled in one or more self-lubricating bearings comprising hardened rolling elements and races, such as balls and ball races sized for close tolerance rotation at very low temperatures and polymeric ball retainers or cages providing lubricity to the retained rolling elements at very low temperatures. While stainless steel rolling elements such as stainless steel balls will work, an improved combination provides ceramic rolling elements, particularly ceramic balls, retained with a polymeric cage for rolling in stainless steel races. An impeller of the pump is mounted to the first end of the shaft, and a casing is mounted to the second end of the shaft and a first magnet is contained within the pump casing mounted to the rotatable shaft. The pump casing is comprised of a material having a coefficient of thermal expansion that is greater than the coefficient of thermal expansion of the material comprising the magnet. The back plate is mounted within a housing having openings formed therein for intake of a fluid to be pumped at low pressure and an exhaust for output of the high pressure fluid and a second magnet is positioned in close proximity exterior to the housing for rotation therearound, the second magnet being adapted for mounting to the drive shaft of a motor or other drive source for rotating the second magnet around the housing, thereby rotating the first magnet within the housing to pump the fluid.